1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus equipped with the same.
2. Description of the Related Art
In recent years, digital cameras that pick up an image of an object using an image pickup element such as a CCD or CMOS sensor have replaced film cameras to become the mainstream. Furthermore, various categories of digital cameras ranging from popular-priced compact type cameras to advanced type cameras for professionals have been developed. The present invention focuses, in particular, on the category of popular-priced compact type cameras.
Users of such popular-priced digital cameras generally wish to enjoy easy and simple shooting in various photographing area (shooting situations) anywhere at any time. For this reason, such users favor small size digital cameras, especially cameras that are small with respect to the thickness direction and can be conveniently carried in a pocket of clothes or a bag. Therefore, a further reduction in the size of the taking lens system is demanded. On the other hand, the number of pixels of the image pickup elements have been increasing, and higher optical performance commensurate with the increased number of pixels of the image pickup element is required. In addition, in order to facilitate mass production, it is necessary that the sensitivity of deterioration in optical performance to manufacturing errors occurring during manufacturing and assembling the lens be made small. Zoom lenses having high zoom ratios of 5 to 7 or more have become popular, since they can be used in a wider variety of photographing area. While there is a demand for further increases in the zoom ratio, there also is a demand for increases in the angle of field. To meet such demands, various types of zoom lens systems have been developed.
As prior arts, there is a known type of compact zoom lenses having relatively high zoom ratios, that is, a zoom lens including, in order from the object side thereof, a first lens unit having a positive refracting power, a second lens unit having a negative refracting power, a third lens unit having a positive refracting power and a fourth lens unit having a positive refracting power. Zoom lenses of this type are disclosed in the following patent documents.    Patent Document 1: Japanese Patent Application Laid-open No. 2006-171055,    Patent Document 2: Japanese Patent Application Laid-open No. 11-52244,    Patent Document 3: Japanese Patent Application Laid-open No. 11-6958,    Patent Document 4: Japanese Patent Application Laid-open No. 8-271788,    Patent Document 5: Japanese Patent Application Laid-open No. 2005-326743, and    Patent Document 6: Japanese Patent Application Laid-open No. 2005-78979.